MuPDF is an open source software framework for viewing and converting PDF, XPS, and E-book documents.
There are viewers for various platforms,
several command line tools,
and a software library for building tools and applications.

Viewers

For Linux and Windows there are two viewers. One is a very basic viewer using x11 and win32, respectively.
It has been supplanted by a newer viewer using OpenGL for rendering, which has more features
such as table of contents, unicode search, etc.
We keep the old viewers around for older systems where OpenGL is not available.

Library

There is also a Java library, which uses JNI to provide access to the C library.
The Java classes provide an interface very similar to that available in the
mutool run command line tool.
This Java library also powers the Android SDK.

If you want to build an application for Android, you have several options. You
can base it off one of the existing viewers, or build a new app using the Java
library directly.